In a Long Term Evolution (LTE) system, the random access technology is an important technology of access control of the receiver in the communication system, and the receiver completes the applications for uplink timing synchronous correction, user power adjustment and user resource demands through a random access process.
An uplink random access preamble of the LYE uses a cyclic shift sequence of the Zadoff-Chu (ZC) sequence, and a random access preamble code is derived by selecting different cyclic shifts (Ncs) based on the ZC sequence. A random access subframe is composed of three parts, namely a Cyclic Prefix (CP) part, a preamble sequence part and a Guard Time (GT) part respectively as shown in FIG. 1.
According to the difference of cell coverages, the lengths of demanded CP are different, and the length of preamble and the length of GT are also different. The existing LTE system supports 5 Formats, namely a Format 0, a Format 1, a Format 2, a Format 3 and a Format 4 respectively, and each format corresponds to a different cell coverage. A cell coverage radius is co-determined by the cyclic shift of the sequence and the GT.
Firstly, the cyclic shift decides whether a cell edge user can distinguish different cyclic shift windows, and the selection of cyclic shift must guarantee that the relevant peak values of the preamble sequence and local sequence of the cell edge user fall in a time window corresponding to the cyclic shift, and the length of the time window is TNcs:
            T      Ncs        =                  Ncs        Nzc            ×              T        SEQ              ;
wherein, Nzc is the length of the ZC sequence, with regard to the Formats 0-3, a value of the Nzc is 839, and with regard to the Format 4, a value of the Nzc is 139. TSEQ is the length of the Random Access Channel (RACH) preamble sequence.
The cell coverage radius decided by the Ncs can be obtained according to the following formula:CellRadius1=0.5×TNcs×3×105 km/s.
A time reference reaching the receiver terminal already has a delay of D1 after the downlink synchronization is completed, and a delay of D2 also exists after the receiver sends a Physical Random Access Channel (PRACH) subframe to the base station, and D=D1≈D2 thus the time window TNcs corresponding to one cyclic shift needs to absorb two delays 2D, and the supported cell radius is required to be halved.
In addition, the cell radius is also related to the GT, the length of CP and the length of GT decide that an RACH subframe of the cell edge user will not interfere with the subsequent subframes. Similarly, there exists the problem of uplink and downlink 2D delay, and the calculation formula thereof is as follows:CellRadius2=0.5×TGT×3×105 km/s;
wherein, TGT is the length of guard time.
In conclusion, the cell radius is co-determined by the Ncs and the length of GT:CellRadius=min(CellRadius1,CellRadius2)
According to the above calculation method, the maximum cell radius supported by each of Format 0˜Format 4 is calculated as shown in FIG. 1, wherein Ts is a sampling interval, and Ts=1/30.72 μs.
TABLE 1Cell coverage radiuses of different FormatsSupportedFormatTCPTSEQTGTcell radiusFormat 03168 Ts24576Ts2976 Ts14.5 km  Format 121024 Ts 24576Ts15840 Ts 77 kmFormat 26240 Ts2 * 24576Ts6048 Ts30 kmFormat 321024 Ts 2 * 24576Ts21984 Ts 100 km Format 4 448 Ts4096Ts 614 Ts 3 km
In a limiting case, with regard to the Format 3, when a value of the Ncs is 839, the maximum range of the supported cell is 100 km, and it can be seen that each of the five formats of the existing LTE random access fails to support the over-distance coverage beyond 100 km, and with regard to the over-distance coverage of air line, it is required to support the coverage beyond 100 km or even 200 km.